Various hybrid vehicles and electric vehicles are proposed. The hybrid vehicle has both an internal combustion engine and a motor as drive sources. The electric vehicle has no internal combustion engine and but has only a motor. Among drive systems for driving a motor used as a vehicle drive source, for example, JP 2009-273286A (US2009/0278483 A1) proposes one drive system.
In this proposed drive system, a DC voltage supplied by a high-voltage battery is converted (boosted) into a predetermined drive voltage by a booster circuit, the drive voltage thus generated is converted into a three-phase AC voltage by an inverter, and the three-phase AC voltage is supplied to a three-phase synchronous motor to drive the motor.
The basic configuration of a booster circuit is generally known. As also described in JP 2009-273286A (US2009/0278483 A1), a switching element is turned on and off to repeat accumulating and discharging the energy of a high-voltage battery in and from a reactor (coil) so as to boost a DC voltage. The booster circuit is provided with a capacitor in an output stage thereof for use in output voltage smoothing.
In the drive systems configured as described above, motor torque variation is caused by various factors. For example, sixth-order variation of a motor current period occurs depending on the structure of a three-phase AC motor. Also, to control the inverter included in such a drive system, it is necessary to detect the current supplied to the motor using a current sensor. An offset error of the current sensor and errors of an interface circuit to which the current data detected by the current sensor is inputted can cause periodic torque variation (first-order variation) synchronized with the electric angular period of the motor. Furthermore, second-order torque variation of a frequency two times that of the first-order variation is caused, for example, by gain errors of the current sensor. Of these torque variations, the first-order variation most affects the booster circuit.
The power supplied to a motor (motor power) depends on the product of motor torque and rotation speed. Therefore, when the motor torque periodically varies as described above, the motor power supplied to the motor also varies periodically, thereby causing the voltage inputted to the inverter (i.e., the drive voltage outputted from the booster circuit) to periodically vary.
That is, when the motor torque varies, the drive voltage outputted from the booster circuit correspondingly varies. With respect to the first-order variation in particular, the frequency of torque variation changes corresponding to the rotation speed of the motor (the electrical angular frequency, to be specific). For example, in the case of a three-phase AC motor having six pole pairs with a maximum speed of 20,000 rpm, the torque variation frequency changes in a range of about 0 to 2 kHz.
The booster circuit, on the other hand, has a reactor and a smoothing capacitor. That is, the booster circuit includes an LC resonance circuit. Of the reactor and the smoothing capacitor, the smoothing capacitor is used also to reduce the ripple of the current inputted to the motor. The capacitor is thus required to have a capacitance C of, for example, about several hundred μF. As for the reactor, it is required to have an inductance L of, for example, about several hundred μH in order to suppress excess voltage applied to the switching element used for boosting and thereby reduce the cost of the switching element. When a booster circuit with a boost ratio of 1:2 is configured using a smoothing capacitor having a capacitance C of 500 μF and a reactor having an inductance L of 300 μH, the resonance frequency of the booster circuit is about 205 Hz as calculated using the following expression (1).(½)×[1/{2π√(500μ×300μ)}]≈205  (1)
The resonance frequency falls in the variation range (0 to 2 kHz) of the torque variation frequency of the motor described above as an example. Hence, during a vehicle travel, the torque variation frequency and the resonance frequency of the LC resonance circuit often coincide with each other. When the torque variation frequency and the resonance frequency happen to coincide with each other, the variation width of the drive voltage outputted from the booster circuit is magnified by the resonance.
Generally, conventional booster circuits have a feedback control function used to perform feedback control in which an output voltage is fed back for comparison with a target voltage and duty control is applied, for example, using a PI (proportional and integral) control unit, to a switching element so as to make the output voltage match the target voltage.
It is, however, difficult to suppress, using such known feedback control alone (for example, using a PI control unit alone), the output variation of a booster circuit caused by motor torque variation. To suppress large output variation caused by resonance, a smoothing capacitor to be used is required to have a large capacitance C. It is, however, unrealistic to use a smoothing capacitor with a large capacitance C so as to suppress the output variation of a booster circuit caused by motor torque variation. A larger smoothing capacitor eventually makes the drive system as a whole costly and larger. This problem is not limited to a booster circuit for boosting a DC voltage. The same problem may occur also with a step-down circuit for lowering a DC voltage. That is, it may occur in any converter whose output power periodically varies by being affected by a load (motor) and which has an internal LC resonance circuit.